[cfe-dev] libc++ supporting older compilers

Fri Dec 18 15:54:35 PST 2015

Since Chandler and Eric are both pinging me: Android is using GCC 4.9 for
all targets (and is very near not using GCC at all), and we're not usually
far behind ToT clang. We also don't care about pre-C++11 support. The
platform is C++14 and NDK users that want C++03 can use stlport.

On Fri, Dec 18, 2015 at 3:50 PM, Chandler Carruth via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> FWIW, I'm not aware of any users here at Google who care about anything
> before GCC 4.9. But we very much would like to have libc++ continue to
> support GCC 4.9. =]
>
>
>
> On Fri, Dec 18, 2015 at 3:22 PM Eric Fiselier via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> Looking at the test-suite results I would say we have trouble supporting
>> pre-4.9. But GCC 4.7 and 4.8 are bad implementations of C++11.
>> GCC 4.9 just works.
>>
>> Libc++ 3.8 should require GCC 4.9 or newer because that's what it
>> actually needs. A C++11 feature complete compiler.
>>
>> Below is a summary of how each GCC version does against the test-suite:
>>
>> C++11 Test Suite Results
>> ----------------------------------------
>> GCC        | Passes   | Failures
>> ----------------------------------------
>> 4.6          | 719           | 4212
>> 4.7          | 4661         | 126
>> 4.8          | 4015         | 772
>> 4.9          | 4775         | 12  * 10 are missing "is_trivially_" type
>> traits
>> 5.3          | 4787         | 2
>> 6.0          | 4787         | 2
>> --------------------------------------
>> Clang 3.2 | 4786         | 3
>>
>> I don't want to claim support for 4.7 or 4.8 unless It's tested regularly
>> and the test-suite passes and that involves fixing hundreds of failures.
>> Some of the failures are due to mis-configuration. However a significant
>> amount are caused compiler bugs.
>> "Fixing" these will involve taking good C++11 and replacing it with w/e
>> the bad compiler accepts.
>> Unless people absolutely *need* this I would be hesitant to do it.
>>
>> C++11 support is only half the story. Even with GCC 4.9 there are 600
>> failures in C++03 mode.
>> Libc++ depends on C++11 extensions in C++03 mode but GCC doesn't provide
>> these.
>> Does libc++ need to support GCC in C++03 mode?
>>
>> C++03 | Passes     | Failures
>> -------------------------------------------
>> 4.6      | 4007         | 648
>> 6.0      | 4029         | 628
>>
>> I'm not trying to artificially limit support but I am trying to keep it
>> moving forward. With limited resources
>> I want to focus on support for C++1z and new libraries like filesystem.
>> Most libc++ users use clang,
>> and clang gave us full C++11 support in 3.0.It's time we demand it from
>> other compilers.
>>
>> The full results of the runs can be found here:
>> http://efcs.ca/libcxx/gcc-46-cxx03.txt
>> http://efcs.ca/libcxx/gcc-46-cxx0x.txt
>> http://efcs.ca/libcxx/gcc-47-cxx11.txt
>> http://efcs.ca/libcxx/gcc-48-cxx11.txt
>> http://efcs.ca/libcxx/gcc-49-cxx11.txt
>>
>> On Fri, Dec 18, 2015 at 9:06 AM, Craig, Ben via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>>
>>> Is there one specific feature though?  Here's the list of features and
>>> releases: https://gcc.gnu.org/projects/cxx0x.html
>>> The thing that looks the most inconvenient to work around in 4.6 vs. 4.7
>>> would probably be template aliases, and even that doesn't seem too horrible
>>> for c++03.
>>> Going from 4.6 to 4.5 would result in the loss of nullptr and constexpr,
>>> and I can see those causing a heavy burden.
>>> If you go the other direction though, you could argue that 4.8 would be
>>> beneficial, as then you get access to TLS.
>>>
>>>
>>> On 12/18/2015 9:56 AM, Joerg Sonnenberger via cfe-dev wrote:
>>>
>>>> On Fri, Dec 18, 2015 at 08:30:26AM -0600, Craig, Ben via cfe-dev wrote:
>>>>
>>>>> This isn't exactly the question that is being asked, but why GCC 4.7?
>>>>>
>>>> Lack of many C++ language features before GCC 4.7 is quite neutering.
>>>>
>>>> Joerg
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>>>
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>>
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